I think that to a certain extent it depends on what the image is of.

If it can be dated (fashion of clothes, computer stuff, etc.) then sales will probably decline to nothing as fashion/technology changes.

But I've found that even 'ageless' images seem to decline. A photo of mine of the first page of Genesis was getting 14-15 downloads per month when I first posted it. Now, nearly 2 years later it's down to 7 per month (though I've tweaked the keywords to try to bring it up again).

Then again ... Shutterstock is different. Newer photos sell better there because of the configuration of their search engine.

And how many to upload at a time? That's like asking 'how long is a piece of string?' As many as you can manage/want/are allowed. In general, the more you upload, the more you sell.

Since most sites sell newer images better, are you able to re-upload your images so they show up in the newer category or would that be against some policy? Or even make a difference at all? Just curious